In the modern communications network space, signal reach and spectral density are important factors in overall network cost. Assuming other factors to be equal, increases in either signal reach or spectral density tend to reduce overall network cost and are thus very attractive to network service providers.
Signal reach is the distance that an optical signal can be transmitted through a fiber, before conversion to electronic form is required to perform signal regeneration. Using suitable optical amplifiers and optical processing techniques, between 10 and 20 fiber spans (of 40–80 km each) can be traversed by an optical signal before optical/electrical conversion and regeneration are required.
Spectral density, which is normally expressed in terms of bits/sec/Hz (b/s/Hz), is a measure of the extent to which the theoretical maximum bandwidth capacity of an optical channel is utilized. This value is generally determined by dividing the line rate (in bits/sec.) of a channel by the optical frequency (in Hz) of that channel. A spectral density of 1 indicates that, for a given channel, the line rate and optical frequency are equal. Existing telecommunications systems commonly operate at line rates of approximately 2.5 Gb/s to 40 Gb/s. At a line rate of 10 Gb/s, current Wavelength Division Multiplexed (WDM) (or Dense Wave Division Multiplexed (DWDM)) transmission systems achieve a spectral density of approximately 0.1 b/s/Hz. If the line rate is increased to 40 Gb/s, the spectral density increases to approximately 0.4 b/s/Hz, illustrating the advantages of increasing the line rate.
However, increasing the line rate raises a number of difficulties. In particular, at line rates of about 10 Gb/s and higher, the physical (e.g. electrical and optical characteristics) of signal processing equipment and optical fiber have an increasingly important effect on the signal-to-noise ratio at the receiving end of a link, which manifests itself as an increased bit error rate of the regenerated signal. For example, optical signal generators (e.g. lasers and modulators) and optical multiplexers are subject to manufacturing tolerances, which may result in signals in some channels being weaker than signals in other channels. These manufacturing tolerances may also introduce noise into one or more channels, which may contain harmonics of the line rate. Furthermore, it is known that optical cross-talk between adjacent channels does not affect all wavelengths equally, so that some channels may be more prone to optical cross-talk than others.
One known method of addressing these difficulties is to interleave a plurality of parallel FEC encoded sub-streams (e.g. having a line rate of 2.5 Gb/s), into a high-speed signal for transmission through the data link. The sub-streams can be FEC encoded (e.g. using a conventional BCH) to form respective FEC blocks (e.g. of 4608 bits in length), in a manner known in the art. Interleaving FEC blocks has the effect of distributing bits of each sub-stream within the high-speed signal. This, in turn, distributes the effects of noise among the sub-streams, and thereby reduces the probability that the error-correction capacity of the FEC encoding will be exceeded for any one channel.
However, this method results in a recurring harmonic relationship between bits of each sub-stream within the high-speed signal. For example, using a conventional bit-wise interleaving pattern, to interleave 16 sub-streams, one bit of each sub-stream occupies every 16th bit of the high-speed signal. Furthermore, a bit of, for example, sub-stream No. 6 always lays between corresponding bits of sub-streams Nos. 5 and 7. This situation renders the high-speed signal vulnerable to patterning of signal degradation, which tends to produce repeating localized bursts of errors within the high-speed signal. Such patterning of signal degradation may, for example, result from noise introduced into the high speed signal by the interleaver, and having a frequency that is an harmonic of the interleaving frequency.
In general, the effects of noise become progressively more pronounced as the frequency of a noise component more closely approximates an harmonic of the high-speed signal, as this tends to produce signal degradation patterning characterized by localized error bursts concentrated within only a few of the sub-steams. This is undesirable because the error rates in the affected sub-streams may be too high to be corrected by the FEC encoding, while unaffected sub-streams will have underutilized error-correction capabilities.
Accordingly, a method and apparatus enabling increased signal reach by distributing the effects of recurring error bursts among interleaved sub-streams, remains highly desirable.